1. Field of the Invention
The present invention relates to an apparatus for improving the transmission rate of encoded data and, more particularly, relates to an apparatus for improving the telemetry transmission rate through a lengthy armored well logging cable of manchester encoded data obtained in a well logging operation.
2. Discussion of the Prior Art
Data acquisition techniques employed in well logging operations produce extensive quantities of data which must be transmitted from borehole encoding equipment to a surface located data discriminator/decoder by way of an armored well logging cable. Because of the extensive distances typically employed in the transmission path, e.g., 25,000-35,000 feet, electrical band pass characteristics of the armored cable employed for transmission place severe electrical constraints on the speed with which data can be transmitted. Typically, the lengthy cable attentuates higher frequency components of the data signal to a greater degree than lower frequency components, making uniform amplitude detection of all frequency components difficult. Increasing the data speed increases the frequency of the various components of the encoded data signal and thus decreases reliability of data detection.
Various attempts have been made to increase the speed of data transmission in the environment just described, including the proper use of line terminating impedances and the use of sophisticated data transmitters and receivers, with some limited success. However, even with these techniques, some of which are very complex and expensive, it has been difficult to obtain reliable data transmission rates of over 40K bits/sec. This is particularly true with manchester encoded data, which has widely dispersed frequency components at, for example, frequencies of f.sub.c, f.sub.c/2, and f.sub.c/3 or f.sub.c/4. These widely dispersed frequency components are distorted to different degrees by the bandpass characteristics of the armored cable, making their detection and proper decoding particularly difficult.